In such machines it is known that, in the course of a thread-winding operation, the speeds of the components involved in such an operation may have to be progressively changed (increased and/or decreased) in order to preserve a certain quality of the product. Thus, for example, the maintenance of a constant rate of twist in the thread being wound up requires that an invariable ratio be established between the rate at which the thread is fed to a traveler from a supply spool driven by an upstream motor and the rate at which the thread is being picked up by the aforestated take-up bobbin on a spindle driven by a downstream motor. This may call for a progressive acceleration of rotation of the supply spool, whose diameter continuously decreases, accompanied by a progressive deceleration of the spindle carrying the take-up bobbin whose diameter continuously increases. In some instances, though, it may be necessary to accelerate the spindle rotation as well as the turning of the supply spool in order to keep the ballooning thread under a certain tension; this is true, for example, with machinery for the winding of glass fibers as disclosed in commonly owned U.S. patent application Ser. No. 480,868 filed Mar. 31, 1983 by Gerd Hausner et al.
Situations also exist, however, where the speed ratio between an upstream motor and a downstream motor of a given winding station may be left approximately constant, e.g. where the thread is being supplied by a tension frame and the variation in the diameter of the take-up bobbin results in only insignificant changes of the twist rate. See, in this connection, my copending application Ser. No. 589,408 filed on Mar. 14, 1984.
There are, of course, several known ways of modulating a motor speed in a programmed manner. French patent No. 1,456,011, for example, discloses two supply networks of different frequencies for the alternate energization of individual a-c motors along with a timer switching each motor from one network to the other and vice versa in a predetermined sequence. Such a system, however, is limited to two discrete motor speeds. German printed specification No. 1,117,716 proposes the use of a main supply network of fixed frequency and an ancillary supply network whose frequency is increasable from a low value to that of the main network, the ancillary network being connectable to the individual motors in a startup period to bring them up to operating speed. This system does not enable a progressive speed modification during a normal operating period.